The invention relates to a vehicle power steering system with an electric motor as well as to a method for controlling an electric motor of such a power steering system.
Power steering systems with an electric motor are frequently also referred to in brief as electric steering systems and are widespread, particularly in vehicles of the lower category (subcompact cars). Increasingly, however, they are also used in vehicles of the compact and full-sized categories. Usually, these power steering systems have an electronic control, which controls the electric motor dynamically, so that the latter, in addition to the steering-wheel torque applied by the driver of the vehicle, generates a motor torque supporting the steering-wheel effort. In comparison to hydraulically assisted power steering systems, which are also referred to in brief as hydraulic power steering systems or servo steering systems, electric steering systems have the advantage that the electric motor can be controlled very accurately by means of a control system. Generally, the known control systems are processor-controlled circuits, which can be programmed so that the control of the electric motor is adapted to the particular requirements and the behavior of the electric steering, can be adjusted very flexibly. The motor torque supporting the steering-wheel effort usually is transferred to the steering mechanics, comprising a gear rack or a recirculating ball nut, over a transmission connection, preferably over a worm gear. It has turned out that the steering mechanics, including the transmission connection, cannot be constructed completely without backlash or, at least, completely without elasticity, perhaps at the worm gear. Under certain driving conditions, this backlash or elasticity leads to chatter, so that a mechanical, generally high frequency vibration can arise and caused an unpleasant, disturbing noise. This state of the phase occurs, above all, when the adhesion of the vehicle to the roadway is only slight under conditions of low friction, as is the case especially when the roadway is wet or covered by ice (glazed frost).
The JP-A62184967 discloses a power steering system with an electric motor, for which the voltage, supplying the electric motor (voltage V4), is specified by the electronic control system. As long as the steering wheel is kept neutral by the driver, that is, as long as the steering-wheel torque is zero, the supply voltage is also set to zero, so that the motor does not produce a torque. As soon as the steering wheel is moved in the positive direction, that is, as soon as there is a positive steering-wheel torque, the supply voltage is generated and, to begin with, held by a delay circuit, at a low voltage (VO) for a particular period of time (TO) in order to suppress the chatter brought about by the transmission (reduction gear 9). This measure takes place always and independently of the driving situation, that is, also in the normal state, when the friction values are large enough and there is no chatter. However, the behavior of the steering system in the normal state is made worse unnecessarily by the delay, which is always provided.